Internet tools such as email, weblogs (blogs), and message boards have grown to become important tools of electronic communication. The growth of these tools, however, has also led to the growth of unsolicited electronic communications known as spam. Spam is disliked by many users and administrators of these communication tools for intruding upon users' privacy and potentially compromising security.
Spam can be controlled at the recipient side or at the creator side. Ways of controlling spam at the creator side include banning users, throttling the number of messages or posts that can be sent or posted, or presenting challenges to verify that the creator of a message is human. Sometimes, these methods can be applied too aggressively, due to uncertainty regarding whether the content in question is spam; the control measures are applied to legitimate users as well as spammers. This can have the effect of inconveniencing legitimate users and making the user experience less productive and enjoyable.